This invention relates to water-developable positive acting lithographic printing plates, and to a photosensitive, photopolymerizable composition for forming the hydrophilic portion of such a printing plate.
Positive acting water-developable lithographic printing plates having a photopolymerizable light-sensitive layer are known in the art. Such plates are described, for example, in U.S. Pat. Nos. 4,147,549 and 4,198,236. More particularly, these plates are prepared from positive-working photosensitive elements which comprise a support, a photosensitive layer or stratum and may have a strippable protective cover layer. The photosensitive layer contains an oleophilic polymer binder which is substantially insoluble in the water developer, a water soluble, ethylenically unsaturated monomer capable of undergoing photoinitiated addition polymerization to form a hydrophilic polymer which is substantially insoluble in the water developer, and an effective amount of a polymerization initiator activatable by actinic radiation and inactive thermally below 85.degree. C. To prepare a planographic printing plate useful in offset lithography, the element is exposed to actinic radiation through a process positive transparency, the protective cover layer is stripped away and the surface of the exposed stratum is washed with water which leaches monomer from the unexposed areas of the stratum. The resulting stratum contains a positive image made up from an oleophilic ink receptive surface from which the monomer has been leached and a complementary hydrophilic surface comprised of the hydrophilic polymer formed. U.S. Pat. Nos. 3,867,147, 3,085,008, and 3,419,394 relate to lithographic plates comprising an oleophilic layer overcoated with a water-soluble hydrophilic material mixed with a diazo compound, said diazo compound being light sensitive and decomposable so as to effect crosslinking in the hydrophilic material. Upon curing, the hydrophilic polymer becomes water insoluble but remains hydrophilic.
Although the above-mentioned patents are several years old, as yet, no positive acting water-developable lithographic printing plate is commercially available because wet strength, scratch resistance, and solvent resistance are features for which problems still exist in the current art. Moreover, the state of the art requires gumming or similar types of post treatment or special fountains in order to maintain hydrophilicity of plates after development.